Sequence of 23 images of Comet 67P/Churyumov–Gerasimenko taken with Rosetta's OSIRIS narrow-angle camera on 4 July 2015, about a month before the comet’s closest approach to the Sun. The three-colour images are made from observations at 480, 649 and 882 nm.
The images are taken at 30 minute intervals and span a full day at the comet, which spins around its axis in about 12.4 hours. The Sun is towards the top of the frame.
The sequence reveals daily colour variations on the surface, with bluer portions of the surface being richer in water ice than their redder surroundings.
A daily cycle of water ice occurs at the comet: quickly turning into water vapour when exposed to sunlight during the local daytime, it condenses back into thin layers of frost and ice as the temperature decreases after sunset, only to sublimate again on the following day.
For more information see: Icy surprises at Rosetta's comet